1. Field of the Invention
Embodiments of this invention generally relate to synthetic microspheres and processes for manufacturing the microspheres. These embodiments have been developed primarily to provide a cost-effective alternative to commercially available cenospheres.
2. Description of the Related Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Cenospheres are spherical inorganic hollow microparticles found in fly ash, which is typically produced as a by-product in coal-fired power stations. Cenospheres typically make up around 1%-2% of the fly ash and can be recovered or “harvested” from fly ash. These harvested cenospheres are widely available commercially. The composition, form, size, shape and density of cenospheres provide particular benefits in the formulation and manufacture of many low-density products.
One of the characterizing features of cenospheres is their exceptionally high chemical durability. This exceptionally high chemical durability is understood to be largely due to the very low content of alkali metal oxides, particularly sodium oxide, in their composition. Accordingly, low-density composites produced from harvested cenospheres usually have the desirable properties of high strength to weigh ratio and chemical inertness. Chemical inertness is especially important in Portland cement applications, where relative chemical inertness plays an important role in achieving highly durable cementitious products. Thus, harvested cenospheres have proven to be especially useful in building products and in general applications where they may come into contact with corrosive environments where high chemical durability is desirable.
Despite the known utility of harvested cenospheres, their widespread use has been limited to a large extent by their cost and availability. The recovery of cenospheres in large quantities from fly ash is a labor intensive and expensive process. Although it is possible to increase the recovery of cenospheres from fly ash by modifying the collection process, the cost of improved recovery does not make this economically viable.
It may also be possible to alter combustion conditions in power stations to increase the yield of cenospheres in fly ash. However, combustion conditions in power stations are optimized for coal-burning rather than cenosphere production. It is not economically viable to increase the yield of cenosphere production at the expense of coal-burning efficiency.
Several methods for producing synthetic microspheres have also been developed and are described in the prior art. Early methods for manufacturing hollow glass microspheres involved combining sodium silicate and borax with a suitable foaming agent, drying and crushing the mixture, adjusting the size of the crushed particles and subsequently firing the particles. However, these methods suffer from the use of expensive starting materials such as borax. Hence, the resulting microspheres are necessarily expensive. In addition, the product has poor chemical durability due to the presence of a relatively high percentage of sodium oxide in the resulting glass composition.
U.S. Pat. No. 3,752,685 describes a method of producing glass microspheres from Shirasu, a naturally occurring volcanic rock. Upon heating at 800 to 1000° C., finely divided Shirasu forms hollow glass microspheres. However, this method relies on the provision of Shirasu, which is not a widely available starting material.
U.S. Pat. No. 3,365,315 describes a method of producing glass microspheres from glass beads by heating in the presence of water vapor at a temperature of about 1200° C. This method requires the exclusive use of pre-formed amorphous glasses as the starting raw materials.
U.S. Pat. No. 2,978,340 describes a method of forming glass microspheres from discrete, solid particles consisting essentially of an alkali metal silicate. The microspheres are formed by heating the alkali metal silicate at a temperature in the range of 1000-2500° F. in the presence of a gasifying agent, such as urea or Na2CO3. Again, these alkali silicate microspheres suffer from poor chemical durability due to a high percentage of alkali metal oxides.
U.S. Pat. No. 2,676,892 describes a method of forming microspheres from a Macquoketa clay shale by heating particles of the shale to a temperature of 2500-3500° F. The resulting product undesirably has an open pore structure leading to a relatively high water absorption in an aqueous cementitious environment.
U.S. Pat. No. 2001/0043996 (equivalent of EP-A-1156021) describes a spray combustion process for forming hollow microspheres having a diameter of from 1 to 20 microns. However, this process is unsuitable for making hollow microspheres having a diameter similar to that of known cenospheres, which is typically about 200 microns. In spray combustion processes as described in the reference, rapid steam explosion ruptures larger particles thereby preventing formation of hollow microspheres greater than about 20 microns in diameter.
Hence, from the foregoing, it will be appreciated that there is a need for low-cost synthetic microspheres with properties similar to those of natural microspheres harvested from fly ash. There is also a need for synthetic microspheres with acceptable chemical durability suitable for incorporation into fiber cement compositions. To this end, there is a particular need for a low-cost, high yield process of producing synthetic microspheres from commonly available raw materials. It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.